Diene-based unsaturated polymers, for example nitrile butadiene rubbers, also known as NBR, produced through polymerization of acrylonitrile and butadiene are well-known in the art. Processes for copolymerization of acrylonitrile and butadiene are described for example in U.S. Pat. No. 3,690,349 and U.S. Pat. No. 5,770,660. Depending on production conditions such polymers can be obtained as latex in aqueous medium. Unsaturated diene-based polymers such as NBR are used for a variety of purposes in industry, moreover processes for hydrogenating such unsaturated polymers are well-known in the art.
It has been known that carbon-carbon double bonds in diene-based polymers may be selectively hydrogenated by treating the polymer in an organic solution with hydrogen in the presence of a catalyst to produce their saturated polymers which have significantly improved end-use properties. Such processes can be selective in the double bonds which are hydrogenated so that, for example, the double bonds in aromatic or naphthenic groups are not hydrogenated and double or triple bonds between carbon and other atoms such as nitrogen or oxygen are not affected. This field of art contains many examples of catalysts suitable for such hydrogenations, including catalysts based on cobalt, nickel, rhodium, ruthenium, osmium, and iridium. The suitability of the catalyst depends on the extent of hydrogenation required, the rate of the hydrogenation reaction and the presence or absence of other groups, such as carboxyl and nitrile groups, in the polymers.
Hydrogenation of diene-based polymers has been very successful, if organometallic catalysts or some special metal salt catalysts and high-pressure gaseous hydrogen are used. Such success has been realized in solution hydrogenation, bulk hydrogenation and direct latex hydrogenation. For the solution hydrogenation of a diene-based polymer, the polymer is first dissolved in an organic solvent and then hydrogenation is carried out; after the hydrogenation, post-treatment is applied to recycle the organic solvent and to recover the metal catalyst. In this field, there have been already many patents and patent applications filed in this area, such as U.S. Pat. No. 6,410,657, U.S. Pat. No. 6,020,439, U.S. Pat. No. 5,705,571, U.S. Pat. No. 5,057,581, and U.S. Pat. No. 3,454,644.
In direct latex hydrogenation, a catalyst is directly added into the latex of a diene-based polymer for the hydrogenation operation. Many diene based polymers, copolymers or terpolymers are made by emulsion polymerization processes and they are in a latex form when they are discharged from polymerization reactors. Therefore it is very desirable to directly hydrogenate a diene-based polymer in latex form which is receiving increasing attention in the recent decade. Many efforts have been made to realize such a process. U.S. Pat. No. 7,385,010 has disclosed a process of directly hydrogenating diene-based polymer latex by using organometallic catalysts and high-pressure gaseous hydrogen.
In bulk hydrogenation, a catalyst is directly mixed with a diene-based polymer or a catalyst is entrapped into the polymer, and then hydrogenation is applied. U.S. Pat. No. 7,345,115 teaches a process of using an organometallic catalyst and high-pressure gaseous hydrogen to hydrogenate bulk diene-based polymers at a temperature higher than 100° C., in which the polymer is directly mixed with the catalyst particles.
A significant characteristic of the above processes is that they all involve catalysts in which a noble metal is involved, that they all require high-pressure hydrogen and that they may need a relatively long reaction time.
To avoid using noble metals and avoid being operated under high-pressure, significant attention has been paid to the hydrogenation of C═C bonds using hydrazine or a derivative of hydrazine as a reducing agent together with an oxidant like oxygen, air or hydrogen peroxide. The hydrogen source to saturate the C═C bonds is then generated in-situ as a result of the redox reactions in which diimide is also formed as intermediate. In U.S. Pat. No. 4,452,950 the latex hydrogenation is performed using the hydrazine hydrate/hydrogen peroxide (or oxygen) redox system to produce diimide in situ. CuSO4 or FeSO4 is used as a catalyst. U.S. Pat. No. 5,039,737 and U.S. Pat. No. 5,442,009 provide a more refined latex hydrogenation process which treats the hydrogenated latex with ozone to break the cross-linked polymer chains which form during or after the latex hydrogenation using the diimide approach. U.S. Pat. No. 6,552,132 B2 discloses that a compound can be added before, during or after the latex hydrogenation to break crosslinks formed during the hydrogenation using the diimide hydrogenation route. The compound can be chosen from primary or secondary amines, hydroxylamine, imines, azines, hydrazones and oximes. U.S. Pat. No. 6,635,718 B2 describes the process for hydrogenating C═C bonds of an unsaturated polymer in the form of an aqueous dispersion by using hydrazine and an oxidizing compound in the presence of a metal compound containing a metal atom in an oxidation state of at least 4 (such as Ti(IV), V(V), Mo(VI) and W(VI)) as the catalyst. In Applied Catalysis A: General 276 (2004) 123-128 and Journal of Applied Polymer Science Vol. 96, (2005) 1122-1125 detailed investigations relating to the hydrogenation of nitrile butadiene rubber latex via utilization of the diimide hydrogenation route are presented which cover examining hydrogenation efficiency and degree of hydrogenation.
It has been found that there are side reactions at the interphase of the latex particles and within the polymer phase, which generate radicals to initiate the crosslinking of polymers in the latex form. Using radical scavengers did not show any evidence in helping to suppress the degree of gel formation. Although there are methods developed to reduce the crosslinking, the aforementioned diimide route still encounters gel formation problems, especially when high hydrogenation conversion is achieved. Therefore, the resulting hydrogenated rubber mass is difficult to process or is unsuitable for further use because of its macroscopic three dimensional cross-linked structure.